Asteroid Strikes Created Earth’s Oldest Surviving Rocks

Research led by Curtin University researchers has concluded that the Earth’s oldest-known evolved rocks formed four billion years ago when asteroids slammed into the Earth’s crust, causing it to melt.

The research, published in Nature Geoscience (https://goo.gl/X5X7iN), found that the Earth’s oldest granitic rocks, which form part of the Acasta Gneiss Complex in north-west Canada, have compositions that are distinct from those typical of Earth’s ancient continental crust. These differences suggest that they formed through a different process.

Lead researcher Dr Tim Johnson of Curtin University said that the rocks were produced by partial melting of iron-rich hydrated basaltic rocks at very low pressures, equivalent to the uppermost few kilometres of the crust. “The melting of these rocks at such shallow levels is most easily explained by meteorite impacts, which would have supplied the energy to attain the extreme temperatures required for melting,” Johnson said.

“Our computer simulations of asteroid impacts show that not only is this scenario physically plausible, but the region of shallow partial melting needed to form these ancient evolved rocks would have been widespread. Given the predicted high flux of meteorites about four billion years ago, impact melting may have been the predominant mechanism that generated granitic rocks at that time.”

Co-author Prof Phil Bland of Curtin University said that the Hadean and earliest Archean eons, 4.5–3.9 billion years ago, were dominated by a barrage of asteroid impacts that would have caused widespread melting and recycling of the Earth’s surface. “Consequently, there are almost no rocks preserved from Earth’s formative Hadean eon,” he said. “The only known evolved rocks from the Hadean eon are those in north-west Canada, which have chemical compositions clearly distinct from those that dominate ancient continental crust worldwide, suggesting they were formed in a different way.”